I learned a great trick.  Eat dinner at one place then walk a mile or so and grab a pastry and java at a patisserie.  This only works if you eat early.  It is a balancing act as the better restaurants open after most bakeries close shop for the evening.

Another option is to buy chocolates or pastries early in the day and store them in your hotel room for after dinner.  If you can wait, of course.

If you choose a “prix fixe” menu you will be eating dessert right after your fois gras, so try and save room.

At any rate, make sure you walk after eating a rich French meal.  That is key when visiting Paris.  Walking is the best way to make room for more food!